1. Field of the Invention
This invention relates to electric hammers, hammer drills or other types of percussive tools having an idle strike preventive mechanism. More particularly, this invention relates to such a percussive tool in which the idle strike preventive mechanism incorporates an additional mechanism to lessen the impact at the start of normal percussive operation following idling operation.
2. Description of the Prior Art
FIG. 8 shows Japanese Published Unexamined Utility Model Application No. 62-174887, which discloses an idle strike preventive mechanism for use in an electric hammer. In this mechanism, a cylinder 40 contains a reciprocating piston 41, an air chamber 42, and a striker 43 interlocked with the piston 41 via the air chamber 42. An axially slide sleeve 45 is mounted over the cylinder 40 and urged forward (toward a tool bit 47) by means of a compression spring 44. Also, an exhaust port 48 is formed in the cylinder 40 for connecting the air chamber 42 with the atmosphere. According to this construction, when the tool bit 47 is pressed against the ground or a workpiece, the slide sleeve 45 is retracted to a rear position together with an intermediate member 46 which is disposed between the tool bit 47 and the striker 43. The retracted slide sleeve 45 closes the exhaust port 48, thereby creating an air spring within the air chamber 42 (and pneumatically interlocking the striker 43 with the piston 41). On the other hand, during idle strikes, in which the tool bit 47 is not pressed against the ground or when the tool bit 47 is not mounted at all, the slide sleeve 45 advances together with the intermediate member 46, thus opening the exhaust port 48. The air spring created in the air chamber 47 is lost, eliminating the interlock between the piston 41 and the striker 43.
Notwithstanding the capability to break the interlock between the piston 41 and the striker 43, the prior art idle strike preventive mechanism falls short in the following respect.
While idle strikes are prevented by the mechanism, the tool bit 47, the intermediate member 46, the striker 43 are located in a forward position. To restore normal operation from idling operation, the tool bit 47 is pressed against the ground. In this way, the tool bit 47 and the intermediate member 46 are retracted, pushing back the slide sleeve 45, which then closes the exhaust port 48 to place the air chamber 42 suddenly in a sealed condition. Also, the striker 43 is simultaneously pushed back by the retracted intermediate member 46 to be interlocked with the piston 41. The air spring created in the suddenly sealed air chamber 42 causes an abrupt retraction of the striker 43 at the start of its interlock with the piston 41. The striker 43, being abruptly retraced, causes a strong impact or jolt at the beginning of the subsequent percussive operation. The impact is problematic not only because it lowers the operability of the hammer drill, but also because it causes the tool bit to jump on the workpiece and to chip off the edge of the drilled bore.